If you want to do that programatically, you should have a look at org.eclipse.papyrus.uml.diagram.wizards.CreateModelWizard.createAndOpenPapyrusModel(DiResourceSet, IFile, String). If you want to do that from the UI, then you can right click on the .uml file in the project explorer and select New -> Other ... -> New Papyrus Model. Please be aware of bug 317156. There are also some other recent posts in this forum about that topic.

Your app should be an osgi bundle. So you can just require the bundle org.eclipse.papyrus.uml.diagram.wizards in the manifest and have the rest imported as transitive dependencies of that. But since you are going to make this programatically, you won't be able to use those classes directly, since they are RCP UI classes.